1. Technical Field
The present invention relates to a wireless communication apparatus and a wireless communication system that are applicable to a sensor.
2. Background Art
Recently, for the purpose of effective usage of frequency resources, a part of the frequency band of a digital cordless phone becomes available for wireless communication of a sound communication device, such as a wireless intercom or a wireless sensor, or a data communication device. In this frequency band, wireless communication is authorized which is on the basis of a wireless communication method of the DECT (Digital Enhanced Cordless Telecommunications) standard that has been widespread as a communication method of a digital cordless phone in countries around the world, and with a wireless communication apparatus using this frequency band, the use of a device for inexpensive, commercially available wireless communication has been enabled for a DECT type digital cordless phone. Further, from the viewpoint of effective usage of frequency resources and user convenience, there is a need for the development of complex products, such as a digital cordless phone, a wireless intercom, and a wireless sensor.
The DECT standard adopts a TDMA (Time Division Multiple Access)/TDD (Time Division Duplex) method that is configured to include 24 slots (12 slots for uplink and 12 slots for downlink) in one frame having a cycle of 10 ms.
FIG. 1 shows DECT type frame and slot configurations. As shown in FIG. 1, the DECT type wireless communication controls designation of a slot position by assigning numbers, such as Slot: 1, Slot: 2, and the like, for respective slots that perform wireless communication. Further, one frame includes 24 slots that are from Slot: 1 to Slot: 24, and a frame number is given for every frame to perform communication control. One slot is allocated as a control channel, and 11 slot pairs are allocated as call channels. A control signal that is transmitted through the DECT type control channel includes a synchronization signal to achieve bit synchronization and slot timing synchronization, a master unit ID to identify a master unit, a slot number to achieve slot synchronization that is used for the designation of a communication slot or the like, a frame number to achieve synchronization of a frame number that is used for concealment control or the like, and an error detection code to determine existence/nonexistence of errors of control information that includes a destination notice or the like and received data.
The control information that is transmitted to the DECT type control channel is classified into four kinds of messages: an NT message to notify of a master unit ID that is master unit identification information, a QT message to notify of system information, such as a frame number, a master unit function, a communication frequency, a standby frequency, and the like, and to be used as a reference frame for multi-frame control, a PT message to notify of information related to paging, such as call information and phone number information of a called party for calling party number notification, and an MT message to notify of information related to wireless control that is performed in a MAC layer such as startup of a communication channel and handover. Further, the DECT type cordless phone does not transmit information of the contents with each frame every time, but transmits the control information dispersed over a plurality of frames by determining a unit to assume 16 frames as one period and transmitting one kind of message by one unit. A slave unit that is a tributary station performs intermittent reception control to perform reception operation in conformity to a slot for control, to which the control information is transmitted once per 16 frames. As described above, a multi-frame control is performed to divide the control information into the kinds (the NT message, QT message, PT message, and MT message) and to transmit the plurality of frames.
Since the TDMA type wireless communication apparatus is used by mobile communication or a cordless phone that is driven by a battery, many techniques for low power consumption are disclosed. For example, JP-A-5-102900 discloses a battery saving method by intermittent reception using control channel control through spurious frame configuration and variable control of an intermittent reception period. According to the method in JP-A-5-102900, a master unit intermittently performs on/off control of the power supply of a slave unit in a manner that the master unit generates a counter set value signal that designates an on/off period of the power supply and multiplexes the counter set value signal in a time slot of each frame to transmit the multiplexed counter set value signal to the slave unit, and the slave unit loads and counts the counter set value with a counter and makes the power supply of the slave unit itself into an off state until the counter becomes a predetermined value. In addition, JP-A-5-102900 further discloses changing of the on/off period of the power supply depending on time.
On the other hand, a sensor device, which is used for a fire detector or a window opening and closing monitoring device, has been developed to have a wireless communication function that is driven by a battery, in view of easy of the installation, and many techniques for low power consumption are disclosed. For example, JP-A-2005-84803 discloses a technique to suppress power consumption by performing communication through supplying power to a wireless unit only when it is determined that wireless communication is necessary from a sensor device and to improve reliability of the wireless communication by controlling a transmission interval or the number of transmissions. Further, JP-A-2009-259228 discloses a method in which a battery-driven warning device notifies a user of instances of low battery.
Further, JP-A-2007-214898 discloses a technique to make it possible to perform immediate return from a power-saving mode to another state by realizing a power-saving mode through providing of a sleep mode in which data is maintained although access is not possible, and using SDRAM as a save area for information that would cause trouble if it is lost during sleeping of a communication system processor in a portable wireless communication terminal that is divided into an application system processor and the communication system processor.